Hartnett was born in San Francisco, California, but grew up in Saint Paul, Minnesota and attended Cretin-Derham Hall High School there before attending South High School in Minneapolis, Minnesota, from which he graduated in June of 1996. He then attended SUNY in Purchase, New York. By April of 1997, he was offered the role of Michael Fitzgerald in the short-lived American television series Cracker. Hartnett started off doing small plays and national television commercials, but began his break into the big-screen movie business with his starring role in Halloween H2O: 20 Years Later (1998).
- He was raised for the most part by his father, Daniel, and stepmother, Molly. His mother moved back to San Francisco after divorcing his father.
- He quit playing football in high school because of an injured left knee
- He is an ex-vegetarian.
- He was chosen as one of Teen People Magazine's 21 Hottest Stars Under 21 in 1999, one of Teen People's 25 Hottest Stars under 25 and one of People Magazine's 50 Most Beautiful People, both in 2002, and was voted Bliss Magazine's 3rd sexiest male.
- Hartnett shares his birthday with Robin Williams.
- Has three younger siblings: Jessica, Jack, and Joe.
- Was set to play the role of Tino in Deuces Wild but dropped out when he won the role of Danny Walker in Pearl Harbor. The Tino role eventually went to James Franco.
- Dated his childhood sweetheart Ellen Fenster. They had been together for a number of years and about a year ago, they bought a $2.5 million mansion together in Minneapolis. In February 2004 the two decided to go their separate ways.
- Is now dating actress Scarlett Johansson. The couple is looking to move in together in a house in Manhattan.
- The Black Dahlia
- Sin City (2005)
- Mozart and the Whale (2005)
- Wicker Park (2004)
- Hollywood Homicide (2003)
- 40 Days and 40 Nights (2002)
- The Same (2001)
- Black Hawk Down (2001)
- O (2001)
- Pearl Harbor (2001)
- Town & Country (2001)
- Blow Dry (2001)
- Member (2001)
- Here on Earth (2000)
- The Virgin Suicides (1999)
- The Faculty (1998)
- Debutante (1998)
- Halloween H20: 20 Years Later (1998)
- Cracker (1997) TV Series